The long term objective of this proposal is to increase basic understanding of the events of early determination and differentiation in the mouse embryo. The molecular probes proposed for this are highly specific monoclonally produced non-H-2-antibodies. The ontogenetic development of these minor histocompatibility systems may be of significance in cell-cell interaction in developmental processes, and further, the identification and quantitation of genetically determined cell surface molecules that can be visualized on the embryo is of great potential value to experimental embryology, and of potential value for contraception.